Invented in 1816 by Scottish clergyman and inventor Robert Stirling, the early Stirling engines were very large machines used in industrial settings as an alternative to steam engines, which had a history of explosive accidents. These engines are sometimes called “hot air engines” although the working fluid used today usually is one of several different gases including air, helium and hydrogen. The use of a regenerator, a component that acts as a temporary thermal storage buffer, positioned between the hot and cold reservoirs of the engine is one of the primary characteristic that distinguishes a Stirling cycle engine from other engines that rely on thermal energy supplied externally. The regenerator contains a matrix material that is able to rapidly absorb and dispense thermal energy.
Interest in Stirling cycle engines has increased in the last few decades. The desire to reduce the use of fossil fuels has been a driving force. Interest in solar energy, geothermal energy and the use of the waste heat produced as a byproduct of various manufacturing processes has directed attention toward developing improved Stirling cycle engines that might be able to utilize various sources of thermal energy.
Deficiencies of Prior Art Stirling Engines
There are several factors which limit the efficiency of prior art Stirling engines:
In those engines that employ a piston/crankshaft design, during the power cycle, the piston rod that transfers energy to the crankshaft delivers maximum energy when it is tangent to the circular crankshaft. At all other times, only a fraction of the energy is used to rotate the crankshaft. Also, whenever either of the two pistons rods are not moving parallel to the axis of their respective cylinders, transverse forces are exerted on the pistons resulting in excessive friction between piston and cylinder.
Any part of the working fluid that does not participate directly in the expansion and contraction cycles will reduce the efficiency of the engine. This dead space, which is also known as unswept volume, can be in several engine structures but is often located in the conduit connecting the hot and cold reservoirs and includes the regenerator. This can be significant depending on the proximity of the two reservoirs. As this volume increases, efficiency decreases.
Stirling cycle engines containing reciprocating pistons generally have low efficiencies. As the working fluid in the hot cylinder is heated and expands, the length of the cylinder and crankshaft linkage will determine the time that the piston within the cylinder can transfer energy to the crankshaft during a cycle. A longer cylinder, in relation to its diameter, increases the energy transfer. The standard design of the power piston's rod/crankshaft linkage found in most prior art Stirling engines limits the ratio of the stroke length/diameter of the piston, and therefore limits the efficiency of the engine.
Most prior art Stirling cycle engines rely on reciprocating piston technology but there are others that have novel non-piston implementations. Among these are a variety of rotary type Stirling engines including patents: U.S. Pat. No. 7,185,492, U.S. Pat. No. 4,753,073, U.S. Pat. No. 5,335,497, U.S. Pat. No. 4,206,604, U.S. Pat. No. 3,984,981, U.S. Pat. No. 6,109,040 and U.S. Pat. No. 8,495,873. All of these, as does the present invention, have a unique design.
In the ideal Stirling engine, all of the working fluid would alternately be heated and then cooled providing completely separate expansion and compression cycles. There would be no concurrent heating and cooling of the working fluid that might cancel out some of the desired expansion/compression effects. Designers of Stirling engines try to minimize the overlap of the heating and cooling of the working fluid but the fixed piston rod/crankshaft linkage constrains this minimization.
The present invention mitigates the above limitations of prior art Stirling engines.